1. Field of the Invention
The present invention relates to an electrophotographic recording apparatus used in an image forming apparatus such as a PPC (Plain Paper Copier) or a LP (Laser Printer). More particularly, the invention is concerned with a developing unit which has a toner delivering member. The toner delivering member delivers toner to an image supporting member.
2. Discussion of the Background
FIG. 1 is a schematic elevational view showing a conventional developing unit used in an electrophotographic recording apparatus. It includes a drum shaped photosensitive body 1, a developing unit 2, an agitator 3, a supplying member 4, a developing roller 5, a blade 6 for forming a thin layer and a housing 7. When the developing unit 2 is provided in a predetermining position of a electrophotographic recording apparatus, the developing roller 5 comes into contact with the photosensitive body 1. New developer comprising toner and an additive in the developing unit 2 is stirred by the agitator 3. The additive may be zinc stearate (a fatty acid) for preventing toner sticking and silica for enhancing toner fluidity. Developer is delivered to the developing roller 5 by the supplying member 4.
The developing roller 5 has a conductive core body. The surface of the conductive core body is coated with resin, and is magnetized. Developer is attracted to the surface of the developing roller 5 to form a developer layer thereon. The developer layer is limited to a thin layer by the blade 6, and is charged by friction. Frictionally charged toner on the surface of the developing roller 5 is electrically transferred and adhered to an electrostatic latent image formed on the surface of the photosensitive body 1. The electrostatic latent image on the surface of the photosensitive body 1 is developed.
However, in the developing unit shown in FIG. 1, when inversely charged toner is adhered to the developing roller 5, it is transferred to an area outside of the electrostatic latent image's area on the photosensitive body 1. This therefore raises a problem of contamination on the background of the recording paper. As schematically shown in FIG. 2, a developing unit constructed with an intermediate roller 8 and a developing roller 5 has been proposed to solve the above mentioned problem. The intermediate roller 8 has a conductive core body and an elastic surface layer. A charging roller 9 charges a surface of the photosensitive body 1 uniformly and a transfer roller 10 transfers a developed electrostatic latent image to a recording paper (not shown).
The same members or the members having the same function in the developing unit shown in FIG. 1 have the same reference numeral in FIG. 2, and their detailed explanation is omitted here.
In the developing unit shown in FIG. 1, toner on the developing roller 5 is directly attached to the electrostatic latent image on the photosensitive body 1, and the electrostatic latent image is developed. In the developing unit shown in FIG. 2, toner on the developing roller 5 is attached to the electrostatic latent image on the photosensitive body 1 via the intermediate roller 8, and the electrostatic latent image is developed. The developing unit shown in FIG. 2 prevents contamination of the background of the recording paper due to inversely charged toner. As the surface of the intermediate roller 8 is elastic, the electrostatic latent image can be developed without stressing the photosensitive body 1.
FIG. 3 is a chart showing a relationship between the recording paper number and the concentration of the additive in the developing unit in FIG. 2. As shown in FIG. 3, when an endurance test was performed on the developing unit shown in FIG. 2, the concentration of the additive in the developer within the developing unit gradually increased over time. Especially the concentration of the additive at the area P in FIG. 1 (at an approximately lower portion of the blade 6) increased.
Particles of zinc stearate are larger than those of toner, and are charged in inverse polarity to that of toner. The particles of zinc stearate clump to one another so as to become large particles which are charged in inverse polarity to that of the toner, thereby attracting toner. This toner thus cannot be transferred to the photosensitive body 1. Image quality is therefore deteriorated, due to poor printing of the image or contamination of the background.
The additive must therefore be removed from the developer. Two methods for removing additive from the developing unit may be considered:
(a) a removing unit for removing additive from the developing unit is provided in the developing unit. PA1 (b) toner and additive are transferred to the photosensitive body during the developing process, and are collected by the cleaning unit.
In view of compactness and cost reduction for the developing unit, method (b) is preferable.
A cause of the additive concentration increase in FIG. 1 is explained hereinafter. The additive attaches to the surface of the developing roller 5 directly or via water molecules in the air. The additive is not transferred to the photosensitive body 1 and remains on the surface of the developing roller 5. Toner is transferred to the photosensitive body 1 from the developing roller 5. As residual toner and additive are removed from the developing roller 5 by the blade 6, additive concentration at the area P increases.
A cause of additive concentration increase at an approximately lower portion of the blade 6 in FIG. 2 is explained hereinafter. When the additive is strongly attached to the surface of the intermediate roller 8, the additive is not transferred to the photosensitive body 1 and remains on the surface of the intermediate roller 8. The intermediate roller 8 is rotated in the inverse direction to that of rotation of the developing roller 5. Additive is removed from the developing roller 5 at the nip portion between the intermediate roller 8 and the developing roller 5, and remains at an approximately lower portion of the blade 6.
FIG. 4 is a chart showing a relationship between the recording paper number and the aggregation of toner in the developing unit in FIG. 2. In FIG. 4, when the additive concentration in the developing unit 2 increases, toner in the developing unit 2 is aggregated and the fluidity of the toner becomes poor. The toner supplying amount to the intermediate roller 8 decreases while the additive supplying amount to the intermediate roller 8 increases. The toner/additive ratio on the intermediate roller 8 thereby decreases.
FIG. 5 is a chart showing a relationship between the recording paper number and the image quality in the developing unit in FIG. 2. In FIG. 5, level 5 indicates high quality printing in a recorded image. Level 1 indicates poor printing and missing fine lines in a recorded image. In FIG. 5 a recorded image deterioration is exemplified by low toner concentrations in large black areas, and missing of fine lines in the recording image. The recorded image is similarly deteriorated in the developing unit in FIG. 1.
The above mentioned problem is a defect in the one-component toner developing method.